As an emission type electronic displaying device, there is an electroluminescence device (ELD). As elements constituting the ELD, there is an inorganic electro-luminescence element or an organic electroluminescent element (hereinafter referred to also as organic EL element). The inorganic electroluminescent element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element.
An organic electro-luminescent element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several volts to several decade volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
An organic EL element for practical use is required which efficiently emits light with high luminance at a lower power. For example., there are disclosed an element with long lifetime emitting light with high luminance in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound (Japanese Patent No. 3,093,796), an element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (Japanese Patent O.P.I. Publication No. 63-264692), and an element which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190).
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Prinston University (for example, see M. A. Baldo et al., Nature, 395, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made (for example, see M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000) or U.S. Pat. No. 6,097,147).
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to an illuminator.
For example, many kinds of heavy metal complexes such as iridium complexes has been synthesized and studied (for example, see (for example, see S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001)).
An example employing tris(2-phenylpyridine)iridium as a dopant has been studied (for example, M. A. Baldo et al., Nature, 395, p. 151-154 (1998)).
Further, an example employing as a dopant L2Ir (acac) (in which L represents a bidentate ligand, and “acac represents acetyl acetone) such as (ppy)2Ir (acac) (for example, see M. E. Tompson et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)), or employing as a dopant tris(2-p-tolylpyridine)iridium {Ir(ptpy)3}, tris(benzo-[h]-quinoline)iridium {Ir(bzq)3}, or Ir(bzq)2ClP (Bu)3 has been studied (for example, see Moon-Jae Youn. Og, Tetsuo Tsutsui et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)).
A hole transporting material is used as a host of a phosphorescent compound in order to increase emission efficiency (for example, see Ikai et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)).
Various kinds of electron transporting materials are used as a host of a phosphorescent compound, and further doped with a new iridium complex (for example, M. E. Tompson et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)). High emission efficiency is obtained by incorporation of a hole blocking layer (for example, see Moon-Jae Youn. Og, Tetsuo Tsutsui et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)).
An external qauntum efficiency of around 20%, which is theoretically a threshold value, is attained in green light emission, but there is a problem that emission efficiency greatly lowers at high luminance emission, and further, a sufficient emission efficiency is not attained in another color light, where there is room to be improved. An organic electroluminescent element with high emission efficiency is disclosed in for example, Japanese Patent O.P.I. Publication No. 2002-100476. An organic EL element for practical use is required which efficiently emits light with high luminance at a lower power.
Further, an organic electroluminescent element is strongly required which has durability or emission life enough to be applied to a display or an illuminator. Particularly in a display employing many organic electroluminescent elements, an element continuously emitting light and an element intermittently emitting light differ in deterioration degree. For example, when a first still image is displayed for long time, and then a second still image is displayed, the elements deteriorate at portions corresponding to the first still image, resulting in lowering of luminance of the first image, and the first still image is observed as a dark image with luminance lowered as compared with the second still image. That is, such a display has problem called burn-in. In the display displaying 256 gradations, a luminance lowering of 0.4% corresponds to one gradation lowering. This is a serious problem to be solved to meet requirement for a display providing an image with high precision.
In order to overcome the above problem, various proposals have been made. For example, a method is disclosed in for example, Japanese Patent O.P.I. Publication Nos. 2002-313559, 08-236271, and 2002-367771 in which an organic electroluminescent element is subjected to sealing treatment, a method is disclosed in for example, Japanese Patent O.P.I. Publication No. 07-065958 in which a dopant having an appropriate energy level is incorporated in an organic electroluminescent element to enhance durability, and a method is disclosed in for example, Japanese Patent O.P.I. Publication Nos. 2002-198170 in which a desiccant is incorporated in a space enclosed with a substrate, an organic electroluminescent element and a sealing agent. Further, an attempt has been made which employs an electron transporting material or a material used in a light emission layer each having higher durability in an organic electroluminescent element. This attempt is disclosed in many literatures, for example, in Japanese Patent O.P.I. Publication Nos. 2002-363227, 2002-352961, 2002-356462, 2002-36350, 2002-8860 and 2002-203683).
Generally, when continuous light emission from an organic electroluminescent element is carried out, a decaying speed of its luminance is not constant, but is high particularly at initial emission stage. Accordingly, in a display or an illuminator employing such an organic electroluminescent element, display image quality or illumination ability rapidly lowers at initial stage of operation and after that, it slowly lowers at a relatively small speed. With respect to the “burn-in” described above, this phenomenon implies the fact that images shown earlier on the display is more likely to deteriorate. In a display in which a still image have continued to be displayed at the beginning of operation or in a display for displaying mainly a still image, the “burn-in” phenomenon occurs immediately after the beginning of operation and after that, a viewer watches an image with lowered quality. It is needless to say that this greatly lowers quality as a display.
In order to minimize such a rapid luminance lowering after the beginning of operation and to obtain a reduced and relatively stable luminance lowering speed, an attempt has been made in Japanese Patent O.P.I. Publication Nos. 2002-198172 and 2002-203672 in which an organic electroluminescent element has been subjected to aging treatment until a reduced and relatively stable luminance lowering speed is obtained, as disclosed in Japanese Patent O.P.I. Publication Nos. However, this incorporates a period immediately after the manufacture of the organic electroluminescent element in the form of aging treatment, in other words, a newly manufactured organic electroluminescent element is made fatigue to some extent in advance. However, this method shortens some of life, which the organic electroluminescent element inherently has, in the manufacture, and life of the organic electroluminescent element is considered to be shortened in proportion to degree of the aging treatment. If an organic electroluminescent element can be manufactured which does not greatly lower luminance, the aging treatment being eliminated or if any, being minimized, it can provide an organic electroluminescent element with life which the element inherently has, and an organic electroluminescent element solving or restraining burn-in described above.
In order to improve luminance and emission lifetime of the organic EL element, proposal has been made in which a hole blocking layer, inhibiting migration of holes from the light emission layer, is provided between the light emission layer and the cathode. This hole blocking layer can efficiently accumulate holes in the light emission layer and improve a recombination probability of electrons and holes therein, resulting in light emission with high efficiency. It is reported (see, for example, Japanese Patent O.P.I. Publication Nos. 8-109373 and 10-233284) that a phenanthroline derivative and a triazole derivative are effectively used alone as a hole blocking compound of the hole blocking layer. Further, disclosed is an organic El element with long lifetime in which a specific aluminum complex is used in the hole blocking layer (see Japanese Patent O.P.I. Publication No. 2001-284056).
It has been reported (for example, in Sixty second OyobutsuriGakkai Gakujutsukoen Kai Yokoshu 12-a-M7, Pioneer Gijutsu Johoshi, Vol. 11, No. 1) that a green light emission organic EL element employing a phosphorescent compound, when a hole blocking layer is incorporated in it, exhibits an inner quantum efficiency of approximately 100% and a lifetime of twenty thousand hours. However, there is room to be improved as for emission luminance.
There is an example in which a phosphorescent compound emitting a blue to blue-green color light is used as a dopant compound and a carbazole derivative such as CBP is used as a host compound, but the external qauntum efficiency of this example is around 6%, which provides unsatisfactory results, (see Sixty second OyobutsuriGakkai Gakujutsukoen Kai Yokoshu 12-a-MB), and there is room to be improved. Although emission from a fluorescent compound is employed, an organic EL element emitting blue light with excellent color purity and long life is prepared which employs a carbazole derivative compound in which a linkage group is incorporated in the biaryl position of the center of the molecule (see, for example, Japanese Patent O.P.I. Publication No. 2000-21572). An organic EL element emitting light with further longer life is obtained in which the above compound is employed, a specific metal complex having five ligands is incorporated in the hole blocking layer and a phosphorescent compound is used as a dopant (see, for example, Japanese Patent O.P.I. Publication No. 2002-8860).
However, the carbazole derivative as described in the above patent document in which the linkage group is incorporated as described above does not provide emission efficiency and heat resistance sufficient to be put into practical use. An organic EL element for practical use is desired which efficiently emits light with high luminance at a lower power.